The purpose of this project was to study the nature of the forces through which proteins and nucleic acids recognize one another. The protein examined was pancreatic ribonuclease-A, ribonuclease-S, S-protein, S-peptide and various chemical modifications. The model for the nucleic acid was polyadenylic acid, oligomers of adenylic acid and various derivatives. Interactions were measured by analytical and sucrose gradient ultracentrifugation, as well as by gel chromatography, ultraviolet absorption spectroscopy and fluorescence.